HP 6296A - Page 25

•
•
•
4-S
The
feedback
signals
that
control
the
con-
duction
of
the
series
regulator
are
originated
with-
in
the
constant
voltage
input
circuit
or
the
con-
stant
current
input
circuit.
The
output
voltage
of
the
power
supply
is
sampled
by the
constant
volt-
age
input
circuit
by
means
of the
sensing
termi-
nals
{:S).
The voltage
developed
across
the
cur-
rent
sampling
resistor
is
the
input
to
the
constant
current
input
circuit.
This
voltage drop
varies
in
direct
proportion to the
output
current.
Any
changes
·in
output
voltage/current
are
detected
in
the
constant
voltage/constant
current
input
cir-
cuit,
amplified
by the
gating
and error
amplifiers,
and
applied
to
the
series
regulator in
the
correct
phase
and
amplitude
to
counteract
the
changes.
4-6
The
fault
input
circuit
detects
the
presence
of
overvoltage
or overcurrent
conditic:>ns
and
gen-
erates
the
necessary
tum-down
signals
to
the
SCR
control
circuit
or
the
series
regulator
circuit.
In
the
case
of
an
overvoltage
condition, a
tum-
down
signal
is
applied
to
the
SCR
control
circuit.
The
series
regulator
receives
a turn-down
signal
via
the
gating
circuit
if
an
overcurrent
condition
is
detected.
4-7
The
reference
circuit
provides
stable
refer-
ence
voltages
which
are
used
by the
constant
voltage/current
input
circuits
for comparison
pur•
poses.
The
bias
supply
furnishes
voltages
which
are
used
throughout the instrument for
biasing
purposes.
The meter
circuit
provides
an
indica-
tion
of
output
voltage
or
current
in one of two
ranges.
4-8
SIMPLIFIED
SCHEMATIC
4-9
A
simplified
schematic
of the power
supply
is
shown
in
Figure
4-2.
It
shows
the
operating
controls;
the
ON-off
pushbutton,
the
voltage
pro-
gramming
controls
(R813
and
R814)
and
the
current
programming
controls
(R809
and
R810).
The
METER
switch,
i-ncluded
in
the
meter
circuit
block
on
Figure
4-2,
allows
the
meter
to
read
output
voltage
or
current
in
one
of
two
ranges.
Figure
4-2
also
shows
the
internal
sources
of
bias
and
reference
voltages
and
their
nominal magnitudes
with
an
input
of
115
Vac
and
no
load
connected.
Diode
CR809,
connected
across
the
output
termi-
nals
of
the
power supply,
is
a protective
device
which
prevents
internal damage
that
might
occur
if
a
reverse
voltage
were
applied
across
the
output
terminals.
Output
capacitor,
C809
stabilizes
the
feedback
loop when the normal strapping
pattern
shown
in Figure
4-2
is
employed. Note
that
this
capacitor
can
be
removed
if
an
increas~
in
the
programming
speed
is
desired.
Under
these
con-
ditions,
capacitor
C802
serves
to insure loop
stability.
Resistors
R811
and
R812
limit
the
out-
put
of
the
supply
if
the
straps
between the
output
and
sensing
terminals
are
inadvertently
opened.
4-3
4-10
DETAILED
CIRCUIT
ANALYSIS
4-11
PREREGUIATOR
AND
CONTROL
CIRCUIT
4-12
The
prere9ulator
minimizes
changes
in the
power
dissipated
by
the
series
regulator
due to
output
voltage
or
input
line
voltage
changes.
Preregulation
is
accomplished
by
means
of a
phase
control
circuit
utilizing
SCR's CRS04
and
CRS02
as
the
switching
elements.
The
appropriate
SCR
is
fired
once
during
each
half-cycle
(8.33
millisec-
onds)
of
the
rectified
ac
(see
Figure
4-3).
Notice
that
when
the
SCR
is
fired
at
an
early
point during
the
half-cycle,
the
de
level
applied
to
the
series
regulator
is
fairly
high.
When
the
SCR
is
fired
later
during
the
cycle,
the
de
level
is
relatively
low.
60Hz
AC
INPUT
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-Figure
4-3.
SCR
Phase
Control
of
DC
Input Level
4-13
The
SCR
control
circuit
(See
Figure 4-4)
samples
the
input
line
voltage,
the
output
voltaQe,
and
the
voltage
across
the
series
transistor.
It
generates
a
firing
pulse,
at
the
time
required,
to
fire
the
SCR
so
that
the
voltage
across
input
capacitor
CSOO
will
be
maintained
at
the
desired
level.
4-14
The
inputs
to
the
control
circuit
are
algebra-
ically
summed
across
capacitor
C700.
All inputs
contribute
to
the
time
required
to
charge
C700.
The
input
line
voltage
is
rectified
by
CR704 through
CR707,
attenuated
by
voltage
divider
R700
and
:
R701.
and
applied
to
the
summing
point
at
TP
59
via
capacitor
C700.
Capacitor
C701
is
used
for
smoothing
purposes.
4-15
Transistor
Q702,
connected
in
a common
base
configuration,
provides
a
charging
current
for
the
summing
capacitor
which
varies
in
accord-
ance
with
the
input
signals
applied
to
its
emitter.